The answer is that the distribute-list filters routes in the routing table, not the LSAs sent in OSPF updates. This is because OSPF is a link-state protocol where routers exchange LSAs to build a complete topological database; a distribute-list operates on the local Routing Information Base (RIB), blocking routes from being installed or selected, but it does not alter the LSA database or prevent those LSAs from being flooded to neighbors. On the Cisco CCNP ENARSI 300-410 exam, this concept frequently appears as a trick question testing your understanding of the separation between OSPF’s control plane (LSA flooding) and the data plane (route installation). A common trap is assuming that filtering outbound from the routing table also stops the LSA from being advertised, but OSPF neighbors still receive the full LSA and compute their own routes. Memory tip: “Distribute-list trims the tree, not the seeds”—it prunes what the router uses locally, not what it shares with others.
300-410 OSPF Troubleshooting (v2/v3) Practice Question
This 300-410 practice question tests your understanding of ospf troubleshooting (v2/v3). This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An engineer configures a distribute-list on an OSPF router to filter routes. However, the routes are still being advertised to neighbors. Which is the most likely explanation?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The distribute-list filters routes in the routing table, not the LSAs sent in OSPF updates.
A distribute-list in OSPF filters routes in the inbound or outbound direction of the routing table, but it does not filter the actual LSA advertisements. OSPF uses LSAs to propagate routes; filtering with distribute-list only affects the local routing table, not the LSAs sent to neighbors.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The distribute-list filters routes in the routing table, not the LSAs sent in OSPF updates.
Why this is correct
Distribute-lists in OSPF only control which routes are installed in the local routing table, not which LSAs are advertised.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The distribute-list is applied to the wrong interface.
Why it's wrong here
Even if applied correctly, distribute-lists do not filter LSAs.
✗
The distribute-list uses an incorrect prefix-list.
Why it's wrong here
The prefix-list may be correct, but distribute-lists do not filter LSAs.
✗
The OSPF process needs to be restarted for the distribute-list to take effect.
Why it's wrong here
Restarting OSPF would not change the behavior; distribute-lists do not filter LSAs.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
OSPF Troubleshooting (v2/v3) — This question tests OSPF Troubleshooting (v2/v3) — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The distribute-list filters routes in the routing table, not the LSAs sent in OSPF updates. — A distribute-list in OSPF filters routes in the inbound or outbound direction of the routing table, but it does not filter the actual LSA advertisements. OSPF uses LSAs to propagate routes; filtering with distribute-list only affects the local routing table, not the LSAs sent to neighbors.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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